Dear madogi,
Agree with flogging Lancers!
How much money do you want to spend? By U.S. Spec tables, your Elantra came equipped with either the 1.6 liter or the 1.8 liter engine.
http://en.wikipedia.org/wiki/List_of_Hyundai_enginesThe Beta II 2.0 liter has been written by the US car magazines as an exceptional engine for non-turbo non-premium fuel. If you can afford it, buy a Beta II and swap that WITH the ECM. 140 bhp with US specs. Lots of good features including CVVT on the intake side. Love that engine.
Low on funds? If you have the 1.6 liter, your options are quite limited. SOL.
You are in luck if you have the 1.8 engine however. The Beta 1 1.8 liter can be upgraded to 2.0 liter by changing only the crankshaft (increases piston stroke from 85mm to 93.5 mm. Your horsepower increase is from 130 to 140 hp.
Other cheap performance gains are Cold Air Intake, running 5W20 weight synthetic oil, emptying excess baggage (weight) in the boot, and underdrive pulleys.
If you have money left over...get your ECM reprogrammed, port and polish your intake plenum, get your throttle body bored oversize, and upgrade your exhaust.
The Beta II is easy to upgrade with a turbo in 1.7 years, or you have more money.
Byron